Hydrated oxides, acidic salts, and heteropoly acid salts of many metals (such as titanium, zirconium, hafnium, tin, aluminum, lead, cerium, tungsten, magnesium, manganese, etc.) are effective inorganic ion exchange materials. Inorganic ion exchangers are often stable in high radiation fields, and thus are particularly useful in the removal of radionuclides from waste streams. These inorganic ion exchangers typically have high selectivities and efficiencies for separating and removing fission products, actinides, and other elements from aqueous waste streams. Most of these materials are also compatible with matrices used for long term waste storage, such as in glass, phosphate, or grout. Certain metal oxides are known to be effective catalysts, such as in the photocatalytic decomposition of various hazardous organics. Also, many metal oxides are known to be effective getters for removing volatile fission products from off-gas streams.
Several inorganic exchangers and sorbents are commercially available as pure material in powder or granular form. However, these fine powders and granular particles are often not readily adaptable to continuous processing, such as in column chromatography. Moreover, they often have poor hydrodynamic properties. Some of these powders are fabricated into pellets by using binding materials; however, the binders can lessen the number of available exchange sites. The binders can also block pores and passageways to the exchange sites within the structures and can adversely affect the loading and kinetic behavior of the exchangers.
Another disadvantage of many of the powders, granular materials, and pellets is lack of sorbent reproducibility of the inorganic ion exchangers. These materials are prepared in batch processes in which chemical and physical gradients can occur that cause variances in the crystal morphology and compositions of the products. Also, the granular material may not be very stable and can powder or erode, causing problems in column operations. Additionally, organic binders, when used to make the pellets, are often not stable when exposed to high radiation doses.
Attempts have been made to remedy the problems associated with powders and particles by forming gel particles. There are a number of gel forming processes used in the preparation of inorganic sorbents, catalysts, ceramics, and getters. Common to all these processes is that the constituents of the processes need to be suitable for the bonding of colloidal particles into gel structures. The gels usually are hydrous metal oxides. These processes are generally identified as “sol-gel” processes and the chemistries are complex and path dependent. Typically, they are defined as external or internal gelation processes. In external gelation processes, gelation reactions involve mass transfer to a second phase or fluid. By comparison, there is little or no mass transfer in internal gelation processes.
One of the original external gel processes for the preparation of nuclear fuels was developed at Oak Ridge National Laboratories. It was based on the gelation of colloidal sol droplets by extracting the water from them in an immiscible alcohol. In other external gelation processes, droplets of solutions of organic polymers or sols were chemically gelled with ammonia, usually by mass transfer of the ammonia from a surrounding gas or solution.
Making silica-alumina gel as spheres is an example of one internal gelation process. Gel spheres were made by continuously mixing an acid solution of A1C13 or Al2(SO4)3 with sodium silicate as drops into an immiscible organic medium. The aqueous droplets gelled while in the organic medium. The key to this process was the slow or delayed gelation of silica when the sodium silicate was acidified.
The most widely studied internal gelation processes in recent years involves the water hydrolysis of metal alkoxides. In these processes, solution temperature and pH are key parameters used in controlling hydrolysis and polymerization. However, materials made by the metal alkoxide processes typically are fine powders. Additionally, due to the complex chemistries involved and the difficulty in operating the process, it is difficult to form gel-spheres of hydrous metal oxides wherein the reaction can be controlled and the final product made predictably.